A well known form of anchor for concrete employs a shank having a tapered expansion portion at its inner end. An axially movable or wedge collar formed of two pieces of metal is arranged around the tapered portion and can be expanded outwardly by applying tension along the axis of the anchor. The collar is then wedged outwardly against the concrete or masonry material by the tapered portion.
One form of such an anchor is shown in Canadian Patent No. 766,493.
One of the problems of that type of anchor was the fact that the collar was formed of two or more separate pieces, which were held in position initially by means of adhesive tape or the like. There was always the possibility for the adhesive tape to become dislodged or damaged, and the collars might simply fall off. When introducing the anchor into a bore hole, in other than ideal conditions, there was also a tendency for the collars to become loosened and lost.
A more serious disadvantage was the fact that the anchor is usually used with some form of threaded attachment means at its outer end. This may take the form of a male thread, or may take the form of a socket, these two being the most popular forms of this type of anchor.
In order therefore to apply the expanding tension along the axis of the anchor, it is necessary for something such as a nut, threaded rod or the like to be attached to the outer end of the anchor. However, in this type of anchor there was no structure to prevent rotation of the entire anchor.
If this occurred, then the threaded fastening device at its outer end could only be tightened up with the greatest difficulty.
An improved form of anchor is shown in Canadian Pat. Nos. 899,699 and 953,958.
In this type of anchor the wedge on the collar is formed of a single piece of metal, and the tapered expansion portion is formed with some form of collar engagement surfaces either in the form of a flattened portion, or a bed or button or groove. The collar also had outwardly extending tongues for engaging the sides of the hole in the concrete.
The collar was thus held by the concrete against rotation. The collar in turn through its engagement with the surfaces on the tapered portion, held the shaft against rotation.
This device was a considerable improvement both in the fact that the collar was made of one piece of metal so that it could not become lost, and also because it was much easier to tighten up when a threaded fastening device was attached to the outer end.
However, it is difficult to manufacture the engagement surfaces and the collar to suitable tolerances in existing mass production techniques, and in practice either the tolerances become so slack that the engagement is not sufficiently secure, or alternatively the costs of production rise to the point where customers will not purchase the anchor.
An additional disadvantage of this anchor in practice was found to be the fact that the tongues on the collar tended to become deformed as the anchor was driven into the hole in the concrete. As a result, they no longer had the same holding capacity, so that in certain circumstances it was still possible for the entire anchor to rotate.
For all of these reasons therefore it is clearly desirable to provide an anchor having a more secure form of engagement between the collar and the side walls of the hole, and also having a more secure form of engagement between the collar and the tapered portion of the shaft, so as to positively resist rotation, and at the same time, being capable of being manufactured by simpler mass production techniques, where critical tolerances do not require to be held to the same degree.